Unified Ed25519 authentication - remove credentials system

BREAKING CHANGE: Remove credential-based authentication

- Remove Credentials interface and all credential-related code
- Remove register() method from RondevuAPI
- Remove setCredentials() and getAuthHeader() methods

RondevuAPI changes:
- Constructor now requires username and keypair (not credentials)
- Add generateAuthParams() helper for automatic signature generation
- All API methods now include {username, signature, message} auth
- POST requests: auth in body
- GET requests: auth in query params
- Remove Authorization header from all fetch calls

Rondevu class changes:
- Make username optional in RondevuOptions (auto-generates anon username)
- Make keypair optional (auto-generates if not provided)
- Add generateAnonymousUsername() method (anon-{timestamp}-{random})
- Update initialize() to create API with username+keypair (no register call)
- Auto-claim username for anonymous users during initialize()
- Add lazy getAPI() to ensure initialization

Message format for auth:
- Format: action:username:params:timestamp
- Examples: publishService:alice:chat:1.0.0@alice:1234567890
- Each request generates unique signature with timestamp

Index exports:
- Remove Credentials export (no longer exists)

src/index.ts

@@ -14,7 +14,6 @@ export type {
 } from './types.js'
 
 export type {
-    Credentials,
     Keypair,
     OfferRequest,
     Offer,